Glass ceramics containing mica or glass ceramics containing cordierite as the main crystal phase are known. For example, according to DE-OS No. 2,915,570, because of the low linear thermal expansion coefficient of the cordierite crystals, cordierite-glass ceramics have very good thermal properties, such as, for example, a linear thermal expansion coefficient of 12.times.10.sup.-7 K.sup.-1.
At the same time, in cordierite-glass ceramics, such as, for example, according to U.S. Pat. No. 4,304,603 and DE-PS No. 3,130,977, such advantageous stress ranges can be produced in glass ceramic material that it is possible to produce a material of considerable strengths of 250-300 MPa, whereby tensile strengths, i.e. K.sub.IC values of up to 2.5 MPa m.sup.1/2 are attained. These glass ceramics containing cordierite as the main crystal phase, however, are not workable by machines, i.e., they cannot be shaped by conventional hard metal tools.
As glass ceramic materials with advantageous machineability properties are known micaceous-glass ceramics, for example, according to DE-AS No. 2,133,652, DE-OS No. 2,224,990 and DD-WP No. 113,885.
Because of the easy cleavability of the flat mica sheets and the "card-house arrangement" of the mica crystals in the glass ceramic, good machineability of the materials is possible with hard metal tools. A considerable improvement of the machineability as compared to the glass ceramics with flat mica sheets, such as, for example, DE-AS No. 2,133,652, DE-OS No. 2,224,990, DD-WP No. 113,885, and such glass ceramics which have almost isolated spherical accumulations of fluorophlogopite crystals (such as shown in U.S. Pat. No. 3,325,265) have been attained by the development of a glass ceramic which has a new type of bent fluorophlogopite crystals. Such glass ceramics, as shown in the Patentschrift DD-WP No. 153,108 and subsequent publications, such as Glass Technology 24 (1983) 318, have a structure in which the bent fluorophlogopite crystals measuring 25-100 .mu.m are embedded in optimum concentration in the residual glass matrix and the crystals are touching, so that the best conditions for an optimum machineability are created.
Although DD-WP No. 153,108, has attained an optimum for micaceous-glass ceramics with respect to machineability properties, for a broad range of applications, in particular also for medical uses, of glass ceramics as construction material which can be machined, especially the fracture bending, the hardness, the compressive strength, the wear properties and the linear thermal expansion coefficient of the materials are still insufficient.
In stomatology, as materials for inlays, crowns, tooth constructions and bridges are used preferably precious metals and alloys thereof, vitrified ceramics and organic polymers. However, these materials have considerable shortcomings. Precious metals are expensive, have a high thermal conductivity and their color is not always aesthetically satisfactory. The vitrified ceramics are not machineable with conventional hard metal tools and have disadvantages in the processing due to the occurring shrinkage process. Above all, the organic polymers have an insufficient mechanical strength.
Also the glass ceramics described in the U.S. Pat. No. 4,431,420 and in EP-PS No. 22.655, as well as in U.S. Pat. No. 4,515,634 and DE-OS No. 3,435,348 have considerable disadvantages with respect to the application in stomatology, because their properties are not adapted at an optimum to those of tooth enamel. This is the case, for example, with the tetrasilicic-acid-mica-glass ceramic described in U.S. Pat. No. 4,431,420 and EP-PS No. 22,655, for the linear thermal expansion coefficient which, (according to "The Intern. J. of Periodontics and Restorat. Dent. 2(84) 36") is 72.times.10.sup.-7 K.sup.-1 and which, therefore, is considerably below the value of tooth enamel of 114.times.10.sup.-7 K.sup.-1. Such big differences are the cause of uncontrolled stresses between the tooth and the glass ceramic which, already at the slightest temperature differences, can cause the material to break and can induce the formation of peripheral cracks. Furthermore, the bending strength of this glass ceramic of 50-55 MPa is insufficient for a broad extensive application in stomatology, especially for thin-walled shaped bodies. Another disdavantage of the tetrasilicic-acid-mica-glass ceramic is that, obviously due to the problematic nature of expansion, there cannot be produced any compound material with high-strength parent substances, such as, for instance, corundum or metals.
It is known of glass ceramics containing fluorophlogopite, such as described, for example, in DD-WP No. 113,885 and DD-WP No. 153,108, that they have very good workability properties and parameters such that they are used, for instance, in apparatus construction and machine construction. An application of these materials for stomatologic purposes, however, would lead to considerable disadvantages due to properties which are not optimally adjusted to tooth enamel, for example, thermal properties and hardness.
Besides synthetic resins, there are used predominantly vitrified ceramics as materials for veneer laminates.
Veneer laminates made of vitrified ceramics have been industrially manufactured for the first time after the synthetic resins Hekodent (1935), Paladon (1937/38) and Palapont (1940) were introduced into dentistry as so-called Schroeder's hollow teeth (Cavidens). They serve as outer veneer laminates for artificial teeth made of the above-mentioned synthetic materials in order to compensate for the still insufficient abrasion strength.
Individually produced veneer crowns made of vitrified ceramic were also verifiable at that time, whereby the introduction of the so-called veneer ceramic by Schroeder in 1932 has furthered this veneering technology. As a reaction to the unsatisfactory long-term results of synthetic veneering, during the time period from 1952 to 1969, there was noticeable a preference for veneer laminates made of vitrified ceramic for veneer crowns, whereby there mainly occurred a reversal to finished products made of vitrified ceramic, such as gold-button teeth and platinum long-pin teeth. The system of mineral-faced crowns, which was introduced then, was based on definite end-shapes of veneer bodies which were obtained by grinding out the inside of the above-mentioned full-bodied teeth (Reumuth, E. and E. Arnold: Die Rostocker Facettenkrone. Dtsch. Stomat. 13 (1963) 391-398/Armbrecht, E. and A. Gerber: Die Schweriner Facettenkrone. Zahntechnik (Berlin) 5 (1964) 93-103). With the development and introduction to the market of the so-called metal ceramic, there was established a reduction in veneer technology in the countries with sufficient availability of the metal ceramic, whereas in countries where it was not available, it induced an industrial production of facing veneers made of vitrified ceramic in order to facilitate the work of the dental technician (Richter, H.: die Keracette, eine keramische Zahnschale. Zahntechnik (Berlin) 17 (1976) 313-316). The availability of composite synthetic materials, together with the introduction of adhesive techniques to clinical practice and dentistry led to the introduction to the market of laminates made of vitrified ceramic of a new type which were usually used for "front-side restorations", generally for front teeth, by the dentist himself (Calamia. J. R.: Etched porcelain veneers: The current state of art. Quint. Int. 16 (1985) 5-12).
Vitrified ceramic facing veneers cannot be produced industrially below a minimum thickness of 1.5 to 2 mm. Therefore, they are unsuitable for an outer layer made of ceramic of a compound shell.
S. Hobo, and T. Iwata described a hydroxyl-apatite-glass ceramic which, by means of casting technology can be shaped into veneer laminates for facing visible tooth surfaces (S. Hobo, and T. Iwata, Castable Apatite Ceramics As A New Biocompatible Restorative Material, I. Theoretical Considerations, in Quit.
Int. 16 (1985) 135-141). This glass ceramic is especially explained for an individual production technology and permits only the production of veneer laminates of greater wall strengths, similar to those made of vitrified ceramics, and is not suitable for industrial production technologies and backing it with layers of synthetic resin of compound materials, for instance, in order to improve the state of the color. A utilization of the already mentioned glass ceramics (U.S. Pat. No. 4,431,420, EP No. 22,655, U.S. Pat. No. 4,515,634, DE-OS No. 3,435,348, DD-WP No. 113,885 and DD-WP No. 153,108) is not known for facing shells and the above-mentioned deficiencies would also have a negative effect in case of such use.
As materials for adhesive brackets have been described up to now metal, synthetic resin and vitrified ceramic. At the present, there are used mainly tooth adjustment elements made of metal because they have good mechanical stability and a low friction of the adjustment arch in the slit. In order to attain sufficient bonding strengths for orthodontal adhesives, the preparation of the base surfaces of the tooth adjustment elements made of metal requires expensive and complicated technological methods, such as, for example, according to DE-OS No. 2,618,952, the production of peripheral perforated bases or the creation of retentive network bases, photo-etched spherical indentations, for instance, according to DE-OS No. 2,910,070, or wedge-shaped thread grooves. Because of the utilization of these expensive technologies there can be attained bonding strengths of up to a maximum of 12.3 MPa in adhesive metal brackets (Diedrich, P. and Dickmeiss, B. in Fortschr. Kieferorthop. 44 (1983), 298-310). One of the problems in the utilization of adhesive metal brackets has proven to be that various corrosion phenomena occurred on the brackets and caused permanent discoloration of the tooth enamel which, according to Kappert et al. (Fortschr. Kieferorthop. 45 (1984), 271-283) can be attributed to traces of Cr and Fe. The aesthetically unfavorable effect of metal tooth adjustment elements can be improved according to U.S. Pat. No. 4,527,975 by means of a facing made of tooth-colored synthetic resin which, however, in addition to the considerably greater material and time expenditures for the production and incorporation of this facing also considerably increases the risk of dental caries for the patient. In comparison to metal brackets, synthetic brackets, for example, according to GB-Patent No. 1,506,772, have a much more favorable aesthetic effect which, however, can be strongly reduced because of a discoloration of the synthetic resin due to foodstuff and coffee, tea, cigarettes, etc. Another disadvantage of synthetic brackets, furthermore, is their low mechanical strength which, according to Dietrich (Fortschr. Kieferorthop. 42 (1981), 195-208) demonstrates itself in a lack of torque strength, slit deformation and abrasion or in the breaking of the wing of the brackets, as well as the occurrence of high friction in the slit.
Even if a metal insert is built into the slit, such as, for example, according to DE No. 2,742,896, these disadvantages are only incompletely removed.
Brackets made of ceramic, preferably of Al.sub.2 O.sub.3, as described in DE-OS No. 2,913,509 and the U.S. Pat. No. 4,219,617, combine an aesthetically favorable effect and good mechanical strength and, therefore, are superior to metal or synthetic resin brackets. The manufacture of ceramic brackets, however, proves to be expensive and problematic because, among others, of the uncontrollably occurring shrinkage during the vitrifying process which absolutely necessitates mechanical after-treatment which, however, can only be realized by means of diamond tools. Furthermore, an individual adaptation of the base surface of the ceramic braces to the shape of the tooth, which is required in certain cases, is only possible with difficulty, because the ceramic is only poorly machine due to its structure and the contained crystal phases. It is a further disadvantage of ceramic brackets that in order to attain sufficient bonding strengths, there are required indentations in the base surface of the brackets in order to assure better adhesion of the orthdontic adhesive, which leads to an additional expenditure in the production of the ceramic adhesive brackets.
The use of Al.sub.2 O.sub.3 ceramic or glass ceramic as implant material is known in head and neck surgery. Because of the chemical material composition of their precipitated crystal phases and their structure, these known glass ceramic materials have the disadvantage that they are difficult to work on intraoperatively.
According to DE-OS No. 3,211,211 and DE-OS No. 3,211,209 there is known, furthermore, a combination of bioinert and bioactive materials for prostheses of the auditory ossicle. The disadvantage of this technical solution consists in that, because of the material strength and its bioinert surface coating, it cannot be worked on intraoperatively.
Furthermore, the operating surgeon has to have available a large assortment of various implant bodies corresponding to the anatomical conditions.
Another disadvantage of this known combination in the category of utilization of generally known glass ceramics is that the partial components have different thermal expansion coefficients so that, in particular, during sterilization, there can occur structural damages on the bonding locations. Especially in large-area implant bodies, the different thermal expansion coefficients of the partial components have a disadvantageous effect.